(a) Field of the Invention
The present invention relates a light emitting device and a manufacturing method thereof, and, in particular, to a top-emission nitride-based light emitting device and a manufacturing method thereof.
(b) Description of the Related Art
Generally, a top-emission nitride-based light emitting device includes a p-type nitride-based cladding layer, an n-type nitride-based cladding layer, and a nitride-based active layer interposed therebetween. In the nitride-based light emitting device, light generated in the active layer passes through the n-type or p-type cladding layer to emit.
The p-type nitride-based cladding layer has a low hole concentration to have high sheet resistance. In order to compensate for the high sheet resistance, a thin ohmic contact layer including a nickel (Ni) thin film and a gold (Au) thin film is suggested to be employed.
However, when the light passes through the p-type cladding layer, the light emitting device has low emission efficiency due to the poor transmittance of the Ni—Au thin films and is thermally unstable due to the small thickness of the Ni—Au thin films.
Therefore, transparent conductive oxides such as indium-tin oxide (ITO) and zinc oxide (ZnO) are introduced as the material of the ohmic contact layer.
However, ITO or ZnO forms a schottky contact at an interface to cause great voltage drop and has large sheet resistance such that the operating voltage of the light emitting device is increased.
In the meantime, a structure for emitting light through the n-type nitride-based cladding layer is suggested. The structure includes a reflective p-type ohmic contact layer under the active layer and an n-type ohmic contact layer along with an electrode pad having a small contact area on the active layer so that the emission efficiency may be increased and heat generated during the operation of the light emitting device may be easily dissipated. However, the surface of the n-type nitride-based cladding layer in the above-described light emitting device may be apt to be oxidized due to the heat generated during the operation of the light emitting device, thereby degrading the reliability of the light emitting device. Accordingly, transparent conductive materials that are hardly oxidized are introduced as the material of the ohmic contact layer for the n-type nitride-based cladding layer.
Examples of widely-known transparent conductive materials include transparent conductive oxides such as ITO, In2O3, SnO2, and ZnO and transparent conductive nitrides such as titanium nitride (TiN).
However, when the above-described transparent conductive oxides and nitrides are deposited by conventional techniques including chemical vapor deposition (CVD) and physical vapor deposition such as sputtering, electron beam deposition and thermal deposition, deposited thin films have large sheet resistance. In addition, the transparent conductive oxides and nitrides have workfunction that is small and difficult to be adjusted, thereby forming high contact barrier and resistance.
Moreover, the transparent conductive thin films have high reflectance and absorbance for the light generated in the active layer and have refractive indices higher than air and two-dimensional flat interfaces, thereby further decreasing the emission efficiency of the light emitting device.